java.lang.NoClassDefFoundError indexing huge history branch

I get the following java.lang.NoClassDefFoundError when indexing relatively huge history included branch.

Indexer command failed (return code 1)
Logging filehandler pattern: /opt/pisces/WORKSPACE/temp8/log/opengrok%g.%u.log
java.lang.NoClassDefFoundError: org/apache/lucene/index/MergeTrigger
... 10 more
java.lang.NoClassDefFoundError: org/opengrok/indexer/authorization/AuthorizationEntity$PluginDecisionPredicate
... 40 more

This was working fine until 1.3.16, starting from 1.4.1.
At first, I got "java.lang.OutOfMemoryError: Java heap space" error on 1.4.1.
So, I increased the Java heap memory from "Xmx4096m" to "Xmx6144m & Xms6144m" then I now get this error.
Please let me know if I can I workaround this error. (Should I increase more memory?)
Thank you very much for your help!!!

17.opengrok0.0.log

Java: 11.0.2
Tomcat: 9.0.13
RHEL: 7.6

pisces@sjc-opengrok-17>./ctags --version
Universal Ctags 0.0.0, Copyright (C) 2015 Universal Ctags Team
Universal Ctags is derived from Exuberant Ctags.
Exuberant Ctags 5.8, Copyright (C) 1996-2009 Darren Hiebert
Compiled: Feb 13 2020, 06:52:57
URL: https://ctags.io/
Optional compiled features: +wildcards, +regex, +iconv, +option-directory, +xpath, +packcc

[vladak]

Definitely try increasing heap size first. Sometimes the errors that arise from the lack of memory are fairly non obvious.

[vladak]

This might be related to #3243 - also having troubles when creating history cache for sizeable history. However looking at the log this failed when writing the new configuration file so the cause is probably different.

[vladak]

The second exception when writing configuration file is likely just a fall out from the previous exception:

2020-09-25 04:01:51.715-0700 INFO t31 HistoryGuru.createCache: Creating historycache for /opt/pisces/WORKSPACE/temp8/src/polaris (GitRepository) without renamed file handling
2020-09-25 04:01:51.715-0700 FINE t31 Executor.exec: Executing command [...}
2020-09-25 18:35:31.532-0700 SEVERE t31 IndexDatabase$1.run: Problem updating index database in directory /opt/pisces/WORKSPACE/temp8/data/index/polaris: 
java.lang.NoClassDefFoundError: org/apache/lucene/index/MergeTrigger
	at org.apache.lucene.index.IndexWriter.waitForMerges(IndexWriter.java:2536)
	at org.apache.lucene.index.IndexWriter.shutdown(IndexWriter.java:1098)
	at org.apache.lucene.index.IndexWriter.close(IndexWriter.java:1140)
	at org.opengrok.indexer.index.IndexDatabase.update(IndexDatabase.java:517)
	at org.opengrok.indexer.index.IndexDatabase$1.run(IndexDatabase.java:219)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.ClassNotFoundException: org.apache.lucene.index.MergeTrigger
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:583)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
	... 10 more

The exception was thrown after some 14 hours after the history cache creation process started.

I increased the Java heap memory again to "Xmx8192m & Xms8192m" then this problem was disappeared.